Inductive power transfer (IPT) or inductively coupled power transfer (ICPT) systems are now an accepted form of industrial power supply chosen particularly for stringent environments. An example arrangement for an IPT system 1 is shown in FIG. 1. A track conductor 2 is energised with a current at a frequency typically in the range of 5 kHz to 50 kHz. A pick-up 3 with pick-up inductance L1 intercepts some of the magnetic field created by the track current. The pick-up 3 is tuned with some form of track compensation 4 and the power output is rectified 5 and controlled by a switch-mode controller 6 to produce a DC output which may be used for a variety of purposes.
A good control technique for pick-ups has been found to be provided by decoupling as described in U.S. Pat. No. 5,293,308, which is assigned to the assignee of the present invention and incorporated herein by reference. An arrangement from U.S. Pat. No. 5,293,308 is shown in FIG. 2. Power control may be implemented using a single switch. The output power is directly proportional to (1-D), where D is the duty cycle of the switch. The switch may be switched at a high speed or slowly, depending on the capability of the switch and the requirements for the particular application. At fast switching speeds, the DC inductor LDC smoothes the power flow such that the pick-up circuit and its tuning capacitor operate at a voltage directly proportional to the power output. At slower switching speeds the resonance between inductor L1 and capacitor C1 completely collapses when the switch is turned on for a long time and must be re-established when the switch is turned off again. Ideally the energy in the resonant circuit is maintained in LDC with current fly-wheeling through the switch and the rectifier bridge while the switch is on, but in practice this energy may be lost. Both fast and slow switching have power controlled by the duty cycle of the switch as noted above.
This method of control is easy to implement and has many desirable features. However, this pick-up controller does not operate with a unity power factor reflected back on the track, and the induced voltage in L1 is not in phase with the current in L1. In practice, therefore, the circuit is not as efficient as it might be, as the resistive losses in L1 are always higher than ideal and ultimately set the power limit for the pick-up.
Unity power factor pick-ups may be achieved with series tuned pick-ups but these pick-ups result in a power surge on switch-on that is difficult to control. They are also potentially damaging in a short-circuit as the short-circuit currents that they produce may be of a very large magnitude.
A parallel tuned pick-up which achieves a unity power factor is shown in FIG. 3. This pick-up controller is described in International (PCT) Application No. PCT/NZ2007/000131, which is assigned to the assignee of the present invention. The topology of the circuit of FIG. 3 is similar to that of the circuit of FIG. 2, but the DC inductor of FIG. 2 has been replaced with a substantially smaller AC inductor in FIG. 3. An extra capacitor, C3, has been added to compensate for the reactive loading of the rectifier.
This circuit is a considerable improvement on the circuit of FIG. 2, but at some cost. The circuit of FIG. 3 cannot be switched at a high speed as there is now no DC inductor to smooth the power flow from one switching cycle to the next. Also, the arrangement of FIG. 2 provides for a further switching option of switching slowly but then using a switching “burst” when the switch is turned on. At this time the current through LDC can be monitored and if it becomes too high, the switch may be temporarily switched off to recover this energy to the output capacitor, thereby improving efficiency, achieving a “softer” switching-on for the switch, and reducing circuit losses. These switching options are not possible with the unity power factor circuit of FIG. 3.
Switching characteristics are particularly important for higher power pick-ups and controllers. For example, in a pick-up rated at 25 kW, all 25 kW is turned on and off very sharply when the switch is operated. This may cause disruptions to the 3-phase power supply such that other users are affected. With very high power circuits, fast switching is also not attractive as circuits of this “boost” configuration type are not favoured at higher power levels.